It is known to press parts of metal or plastic together by providing recesses or protuberances on the parts like the tongue and groove principle and pressing the protuberances into the recesses. In addition, it is also known from German Pat. No. 455,259, for example, to furnish the parts being joined together with thread-shaped or conical bore holes, a cylindrical pin being inserted into the respectively coinciding holes.
These methods are disadvantageous to the extent that the surfaces being connected have to be shaped or machined in a certain manner. This leads to considerable expense, especially for mass production, with the result that other methods of joining metal parts such as soldering are preferable. In German document No. 2,264,142, laid open for inspection, the disadvantages and difficulties of the known methods are discussed in the introduction and present a good survey of the present state of the art in the field of composite metals. For the purpose of solving the problems, this publication suggests that the surface of an iron plate be furnished with a multiplicity of indentations of suitable width and depth. Aluminum material is then sprayed into these indentations. The latter are produced mechanically, for example, by means of a tilted cutting tool which is a very expensive process.
German Pat. No. 519,945 suggests a method for the mechanical joining of two metal sheets which lie on each other and have different degrees of hardness. The harder of the two has indentations into which some of the softer sheet flows when the metal sheets are pressed together. This patent specification suggests that the indentations be tooth-shaped. However, this means that the harder sheet has to be again treated in a certain way.
U.S. Pat. No. 3,485,518 teaches the use of metal joining strips by means of which wood boards can be joined together in flush engagement (column 4, line 34). The joining strips have nail-shaped teeth with which they penetrate the surface of the wood. This is a type of nailing with nail strips.
All the methods described so far also have the disadvantage that the parts to be joined together must have a certain minimum thickness - on the one hand, so that the necessary indentations can be worked into the surface, or on the other hand so that the nail strips or joining pins, for example, can penetrate far enough. Consequently, the known methods cannot be employed as a practical matter for the joining of relatively thin sheets of metal.
A method for the cold welding of two pieces of metal is disclosed by Swiss Pat. No. 383,730. By the "cold welding" of two pieces of metal, we mean methods which are carried out at very great pressure and used generally to join different kinds of metal, for example, aluminum and copper and alloys of these metals. Roughening and thorough degreasing of the metal surfaces which are to be joined together are essential for cold welding. The method described in the aforementioned Swiss specification permits the elimination of the "scratching" or "brushing" of the metal surfaces to be joined. To this end, a third piece is placed between the two pieces which are to be welded together and during the pressing process, such third piece exerts a roughening, scrubbing or tearing action on the surfaces of the metal pieces being joined together so that cold welding takes place at the scratched, scrubbed or torn surfaces.
Prior art cold welding processes also include the socalled "explosive plating" techniques. In these techniques two metal plates are held apart a selected distance such as, for example 0.4 mm, by means of spacers. An explosive is applied to the upper plate and ignited. The upper plate is thrown against the lower one by the force of the explosion so that the metal plates are thereby joined. This method is expensive because bunker-like buildings are required for the purpose of protection. Also, the quantity of explosives required is considerable.